1. Field of the Invention
The present invention relates generally to the use of liquified and supercritical gases, hereafter described as dense fluids, for cleaning substrates. More particularly, the present invention relates to a process of using microwave energy in combination with dense fluids or dense fluid solvent mixtures and centrifugal force to simultaneously clean a variety of inorganic and organic substrates, including biomaterials and hazardous wastes, to prepare said substrates for use in biological, high energy, high vacuum, high temperature, or high pressure systems and applications or, in the application of cleaning (decontaminating) hazardous wastes, to prepare said substrates for reuse or for safe disposal. Cleaning in the present invention is defined as removing one or more contaminants from a substrate. More specifically, cleaning substrates using the processes of the present invention is defined as a process of removing one or more contaminants from a contaminated or impure substrate to produce a clean or pure substrate exhibiting improved chemical or physical properties such as increased visual cleanliness, improved electrical insulation, lower thermal outgassing, or biological cleanliness (sterility), among other improved characteristics. In addition, the processes and exemplary devices of the present invention are useful for detoxifying hazardous materials such as spent activated carbon or to enhance conventional supercritical fluid extraction processes such as the extraction of scents and oils from botannicals. The terms `sterilization`, `substrate engineering`, `preservation`, `extraction`, `separation`, and `detoxification` are used in this patent declaration and are regarded as cleaning.
2. Description of Related Art
Conventional substrate cleaning processes using hazardous organic solvents, toxic gases, radiation, and topical biocides are currently being re-evaluated due to problems with environmental pollution, toxicity, inefficiency, or poor performance. The use of toxic, carcinogenic, or mutagenic substances to achieve sterility have been shown to be deleterious to the environment, pose significant health threats (D. Lynch, et al, "Effects on Monkeys and Rats of Long-Term Inhalation of Ethylene Oxide: Major Findings of the NIOSH Study", AAMI, 1984), require strict control, and create hazardous waste disposal problems. Also, conventional cleaning processes may damage or alter substrate performance properties. For example, steam autoclaving may greatly accelerate oxide growth on titanium biomaterials (J. Lausmaa, et al, "Accelerated Oxide Growth on Titanium Implants During Autoclaving caused by Fluorine Contamination", BIOMATERIALS, Volume 6, January 1985) and must be carefully controlled.
In some cases, the cleaning media chemically reacts with interstitial substrate residues to form harmful by-products. For example, toxic-by-products or residual media left on biomedical substrates following conventional cleaning processes using ethylene oxide gas have been shown to adversely impact implant performance (H. Scherer, et al, "Hazards Related to Gas Sterilized Materials, LARYNG. RHINOL, OTOL., 65, 1986).
Additionally, conventional biomaterial substrate preparation processes require a separate pre-cleaning operation prior to sterilization operations to assure complete substrate sterility. For example, in ultraviolet (UV) disinfection processes, bacterial shadowing by substrate structures, cavities, or other contaminants are a great concern (R. Boylan, et al, "Evaluation of an Ultraviolet Disinfection Unit", THE JOURNAL OF PROSTHETIC DENTISTRY, Volume 58, Number 5, November 1987). Since ultraviolet treatment is generally only effective on line-of-sight substrate sterilization applications, complex substrates with intricate geometries must be scrupulously cleaned using conventional cleaning techniques prior to UV sterilization.
Finally, conventional cleaning processes are often performed as separate operations, involving the immersion of, or the application of topical sterilants, disinfectants, and other chemical agents. For example, several physical and chemical sterilization methods are used in industry. These methods include gamma radiation treatment (Ch. Baquey, et al, "Radiosterilization of Albuminated Polyester Prostheses", BIOMATERIALS Volume 8, May 1987), ultraviolet radiation, steam autoclaving, dry heat, and toxic gas sterilization (MICROBIOLOGY, M. Peczar, et al, McGraw-Hill Publishers, 1977, pp 425-423).
Substrates used for biomedical, aerospace, high energy, and high vacuum applications are fabricated from different types of materials having various internal and external geometries. These may be assembled biomedical devices such as medical implants, valves, or artificial joints, or they may be surgical aids such as sponges, tubing, guidewires, and clips, and may be contaminated with more than one type of inorganic, organic, or biological contaminant. These highly complex substrates require precision cleaning prior to use in critical environments such as the human body. Often, assembled devices must be disassembled to accommodate conventional cleaning processes.
Polymeric substrates used in surgical applications, or biomaterials, must be free of organic and inorganic residues and microbiological contaminants to provide maximum biologic adhesiveness (cellular adhesion) and no biologic reactivity (biocompatibility). These substrates must be capable of performing their intended function over prolonged periods in contact with living tissue and body fluids. This is a highly specialized environment of great biochemical complexity. The principle medical uses of polymers include: structural materials, joint replacements, dental materials, medical devices (including tubing for transport of biofluids both inside and outside biological systems), adhesives, and sutures. Residual moisture, monomers, oils, plasticizers, dyes, pigments, and other additives contained on or in unclean substrates can produce harmful side effects such as toxic chemical release through bioreaction, infection, swelling, or complete implant rejection.
Substrates having polymeric composition such as spacecraft electrical wiring, electronic connectors, and gasketing must meet strigent NASA outgassing performance requirements ("Vacuum Stability Requirements of Polymeric Materials for Spacecraft Application", SP-R-0022A). Interstitial contaminants such as moisture, plasticizers, and oils, if not removed, volatilize under conditions of high vacuum or high temperature. These contaminants migrate from substrate cavities and deposit on adjacent surfaces, causing system functional problems such as dielectric loss, changes in optical transmission or reflection characteristics, and thermal transfer changes, among others.
Because conventional substrate cleaning processes are performed as independent steps, often the cleaning procedure re-contaminates the substrate with residues or adversely affects the physical properties and subsequent performance of the bulk material (J. Doundoulakis, D. M. D., "Surface Analysis of Titanium after Sterilization Role in Implant-Tissue Interface and Bioadhesion", THE JOURNAL OF PROSTHETIC DENTISTRY, Volume 58, Number 4, October 1987).
Additionally, conventional sterilization processes only deactivate biological contaminants and do not remove these deactivated residues from the substrate. These residues have been shown to adversely affect the performance of biomaterials following implant operations.
Additionally, conventional cleaning processes are effective only on external surfaces of composite or intricately arranged substrates and provide little or no internal cleaning and sterilization capability. In implant cleaning applications, it is imperative that both external and internal surfaces of substrates be both chemically and biologically clean.
Conventional substrate cleaning processes rely on methods employing hazardous organic cleaning solvents such as isopropyl alcohol (Hohmann et al, "Method and Apparatus for Cleaning, Disinfecting and Sterilizing Medical Instruments" U.S. Pat. No. 4,710,233, Dec. 1, 1987), chlorinated hydrocarbons, and other toxic and flammable compounds. Also, many conventional cleaning agents and processes are generally not chemically compatible with organic materials such as spices and herbs, only clean external surfaces, and do not provide a means of extending the cleanliness of the material once following processing. Additionally, this patent applies to metal tools, and it does provide for the removal of residual sterilizing agents from substrates.
In another example of known art using supercritical fluids as washing agents (Nishikawa et al, "Method of Processing an Article in a Supercritical Atmosphere", U.S. Pat. No. 4,944,837, Jul. 31, 1990), a material is cleaned in a supercritical atmosphere to prepare the material for resist stripping using a supercritical fluid or an admixture solvent. Our research shows that non-energized mono-phasic supercritical fluids are poor cleaning solvents for many typical contaminant removal or chemical agent implant applications. Firstly, a contaminant or chemical agent must be provided the proper solvent environment in order to be transported. This may involve employing one or more dense fluids, fluid states (liquid and supercritical) and chemical agent admixtures which is based upon knowledge of the solubility chemistries of the targeted contaminants or the chemical agents. Secondly, our research shows that adjunct higher energy is required to efficiently and effectively solubilize and transport contaminants from substrates, not provided for in this patent citation. Finally, this patent does not provide a means of altering the chemical or physical characteristics of the bulk substrates, and contaminants contained within said substrates, for utilization in critical environments where different bulk properties (internal and external characteristics) such as long-term sterility, improved ductility, or improved electrical insulation would be desirable or required.
Examples of known art (developed by the inventor of the processes of the present invention) using phase shifting of dense fluids (Jackson et al, "Cleaning Process Using Phase Shifting of Dense Phase Gases", U.S. Pat. No. 5,013,366, May 7, 1991) and photochemical action on dense fluids (Jackson, "Dense Phase Gas Photochemical Process for Substrate Treatment", U.S. Pat. No. 5,068,040, Nov. 26, 1991), a substrate and its unwanted residues are subjected to dense fluid chemistries that have been phase shifted via temperature (or pressure), or altered photochemically, to create the most suitable solvent environments (like-dissolves-like) for various contaminants on substrates. These processes employ ultrasonic energy, temperature and pressure control, and ultraviolet light singularly or in combination to simultaneously remove one or more contaminants from a substrate. These processes are based upon the action of externally applied energy (heat, fluid pressure, light, and sound energy) on the dense fluid to alter the dense fluid chemistry to effect separation of surface or subsurface contaminants from a substrate. Our research reveals that mechanisms and devices used to perform cleaning operations cited in these patents are not generally effective or efficient when processing bulk materials having different geometries such as connector pins and gaskets. The processes of the present invention provide superior methods for decontaminating substrates having various geometries by activating the unwanted contaminants deep within the substrate to facilitate transfer into the surrounding dense fluid environment and separation from the bulk material. The processes of the present invention are unique in that the applied energy source, microwave radiation, selectively activates surface and interstitial contaminants contained on or within substrates and only minimally interacts with the substrate (solid state) and dense cleaning fluids, including carbon dioxide and xenon (non-polar fluids). Alternatively, polar dense fluid mixtures consisting of, for example, 98 % (by volume) liquified carbon dioxide and 2% (by volume) purified water or n-octyl alcohol may be used in conjunction with microwave energy. Microwave energy will be absorbed by the polar component of the dense fluid mixture, enhancing contaminant-removal or biocidal efficiency (n-octyl alcohol or hydrogen peroxide as biocides) in cleaning applications. Thus the processes of the present invention are used to activate the contaminants, the dense cleaning fluid(s), or both.
Finally, previously described known art using dense fluids as cleaning agents does not address the mechanical energy required to efficiently and effectively remove organic, inorganic, and biological contaminants from substrates. For example, mechanical agitation is mandatory in order to loosen and remove tenacious contaminants such as a oily particle matrix. This is particularly important in cleaning applications where bulk substrates are being processed, for example, hundreds of thousands of oily connector pins having various lengths and dead-end holes having entrapped particles and oils. In the known art related to the present invention, dense fluids or dense fluid mixtures are contacted with substrates. These processes require contact of said dense fluids with a substrate and solubilization of a contaminant. These processes are very inefficient because of the limited solute carrying capacity of dense fluids, due in part, to their low viscosity and density. This is particularly true with regards to cleaning bulk materials. To effectively clean bulk substrates such as ground or powdered botannicals, connector pins, machined parts, or other assemblies, the substrates must be evenly exposed to the dense fluid or dense fluid mixtures and energy sources, in this case microwave energy.
The processes of the present invention use a unique variable-speed basket centrifuge cleaning apparatus that provides the following; 1) homogenize the dense fluid cleaning solvent and 2) uniformly expose of the substrates to microwave energy and 3) provide centrifugal force to separate said contaminants from substrates and 4) provide multi-phase cleaning. Thus, the contaminated substrates are homogeneously mixed and contacted with the dense fluid cleaning agents and cleaning energies while be exposed with microwave and centrifugal energies.
Accordingly, there is a present need to provide more efficient and environmentally safer alternate substrate cleaning processes, having broader substrate cleaning applications, which are suitable for use in removing more than one type of contaminant from a variety substrates having complex geometries, densities, and volume.